Controlling Actuators with CoAP
draft-mattsson-core-coap-actuators-05

Abstract

Being able to trust information from sensors and to securely control actuators are essential in a world of connected and networking things interacting with the physical world. In this memo we show that just using COAP with a security protocol like DTLS, TLS, or OSCORE is not enough. We describe several serious attacks any on-path attacker can do, and discusses tougher requirements and mechanisms to mitigate the attacks. While this document is focused on actuators, some of the attacks apply equally well to sensors.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 21, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. Attacks
• 2.1. The Block Attack
• 2.2. The Request Delay Attack
• 2.3. The Response Delay and Mismatch Attack
• 2.4. The Relay Attack
• 2.5. The Request Fragment Rearrangement Attack
• 2.5.1. Completing an Operation with an Earlier Final Block
• 2.5.2. Injecting a Withheld First Block
• 3. Security Considerations
• 4. IANA Considerations
• 5. References
• 5.1. Normative References
• 5.2. Informative References
• Acknowledgements
• Authors' Addresses

1. Introduction

Being able to trust information from sensors and to securely control actuators are essential in a world of connected and networking things interacting with the physical world. One protocol used to interact with sensors and actuators is the Constrained Application Protocol (CoAP) [RFC7252]. Any Internet-of-Things (IoT) deployment valuing security and privacy would use a security protocol such as DTLS [RFC6347], TLS [RFC5246], or OSCORE [I-D.ietf-core-object-security] to protect CoAP, where the choice of security protocol depends on the transport protocol and the presence of intermediaries. The use of CoAP over UDP and DTLS is specified in [RFC6347] and the use of CoAP over TCP and TLS is specified in [RFC8323]. OSCORE protects CoAP end-to-end with the use of COSE [RFC8152] and the CoAP Object-Security option [I-D.ietf-core-object-security], and can therefore be used over any transport.

The Constrained Application Protocol (CoAP) [RFC7252] was designed with the assumption that security could be provided on a separate layer, in particular by using DTLS [RFC6347]. The four properties traditionally provided by security protocols are:

• Data confidentiality
• Data origin authentication
• Data integrity checking
• Replay protection

In this document we show that protecting CoAP with a security protocol on another layer is not nearly enough to securely control actuators (and in many cases sensors) and that secure operation often demands far more than the four properties traditionally provided by security protocols. We describe several serious attacks any on-path attacker (i.e. not only “trusted intermediaries) can do and discusses tougher requirements and mechanisms to mitigate the attacks. In general, secure operation of actuators also requires the three properties:

• Data-to-Data binding
• Data-to-space binding
• Data-to-time binding

"Data-to-Data binding" is e.g. binding of responses to a request or binding of data fragments to each other. "Data-to-space binding" is the binding of data to an absolute or relative point in space (i.e. a location) and may in the relative case be referred to as proximity. "Data-to-time binding" is the binding of data to an absolute or relative point in time and may in the relative case be referred to as freshness. The two last properties may be bundled together as "Data-to-spacetime binding".

The request delay attack (valid for DTLS, TLS, and OSCORE and described in Section 2.2) lets an attacker control an actuator at a much later time than the client anticipated. The response delay and mismatch attack (valid for DTLS and TLS and described in Section 2.3) lets an attacker respond to a client with a response meant for an older request. The request fragment rearrangement attack (valid for DTLS, TLS, and OSCORE and described in Section 2.5) lets an attacker cause unauthorized operations to be performed on the server, and responses to unauthorized operations to be mistaken for responses to authorized operations.

Mechanisms mitigating some of the attacks discussed in this document can be found in [I-D.ietf-core-echo-request-tag] and [I-D.liu-core-coap-delay-attacks]

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Attacks

Internet-of-Things (IoT) deployments valuing security and privacy, MUST use a security protocol such as DTLS, TLS, or OSCORE to protect CoAP. This is especially true for deployments of actuators where attacks often (but not always) have serious consequences. The attacks described in this section are made under the assumption that CoAP is already protected with a security protocol such as DTLS, TLS, or OSCORE, as an attacker otherwise can easily forge false requests and responses.

2.1. The Block Attack

An on-path attacker can block the delivery of any number of requests or responses. The attack can also be performed by an attacker jamming the lower layer radio protocol. This is true even if a security protocol like DTLS, TLS, or OSCORE is used. Encryption makes selective blocking of messages harder, but not impossible or even infeasible. With DTLS and TLS, proxies have access to the complete CoAP message, and with OSCORE, the CoAP header and several CoAP options are not encrypted. In both security protocols, the IP-addresses, ports, and CoAP message lengths are available to all on-path attackers, which may be enough to determine the server, resource, and command. The block attack is illustrated in Figures 1 and 2.

Client   Foe   Server
   |      |      |
   +----->X      |      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x47
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 1 (Lock)
   |      |      |

Figure 1: Blocking a request

Where 'X' means the attacker is blocking delivery of the message.

Client   Foe   Server
   |      |      |
   +------------>|      Code: 0.03 (PUT)
   |      | PUT  |     Token: 0x47
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 1 (Lock)
   |      |      |
   |      X<-----+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x47
   |      |      |

Figure 2: Blocking a response

While blocking requests to, or responses from, a sensor is just a denial of service attack, blocking a request to, or a response from, an actuator results in the client losing information about the server's status. If the actuator e.g. is a lock (door, car, etc.), the attack results in the client not knowing (except by using out-of-band information) whether the lock is unlocked or locked, just like the observer in the famous Schrodinger’s cat thought experiment. Due to the nature of the attack, the client cannot distinguish the attack from connectivity problems, offline servers, or unexpected behavior from middle boxes such as NATs and firewalls.

Remedy: Any IoT deployment of actuators where confirmation is important MUST notify the user upon reception of the response, or warn the user when a response is not received.

2.2. The Request Delay Attack

An on-path attacker may not only block packets, but can also delay the delivery of any packet (request or response) by a chosen amount of time. If CoAP is used over a reliable and ordered transport such as TCP with TLS or OSCORE, no messages can be delivered before the delayed message. If CoAP is used over an unreliable and unordered transport such as UDP with DTLS, or OSCORE, other messages can be delivered before the delayed message as long as the delayed packet is delivered inside the replay window. When CoAP is used over UDP, both DTLS and OSCORE allow out-of-order delivery and uses sequence numbers together with a replay window to protect against replay attacks. The replay window has a default length of 64 in DTLS and 32 in OSCORE. The attacker can control the replay window by blocking some or all other packets. By first delaying a request, and then later, after delivery, blocking the response to the request, the client is not made aware of the delayed delivery except by the missing response. The server has in general, no way of knowing that the request was delayed and will therefore happily process the request. Note that delays can also happen for other reasons than a malicious attacker.

If some wireless low-level protocol is used, the attack can also be performed by the attacker simultaneously recording what the client transmits while at the same time jamming the server. The request delay attack is illustrated in Figure 3.

Client   Foe   Server
   |      |      |
   +----->@      |      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x9c
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
     ....   ....  
   |      |      |
   |      @----->|      Code: 0.03 (PUT)
   |      | PUT  |     Token: 0x9c
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
   |      X<-----+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x9c
   |      |      |

Figure 3: Delaying a request

Where '@' means the attacker is storing and later forwarding the message (@ may alternatively be seen as a wormhole connecting two points in time).

While an attacker delaying a request to a sensor is often not a security problem, an attacker delaying a request to an actuator performing an action is often a serious problem. A request to an actuator (for example a request to unlock a lock) is often only meant to be valid for a short time frame, and if the request does not reach the actuator during this short timeframe, the request should not be fulfilled. In the unlock example, if the client does not get any response and does not physically see the lock opening, the user is likely to walk away, calling the locksmith (or the IT-support).

If a non-zero replay window is used (the default when CoAP is used over UDP), the attacker can let the client interact with the actuator before delivering the delayed request to the server (illustrated in Figure 4). In the lock example, the attacker may store the first "unlock" request for later use. The client will likely resend the request with the same token. If DTLS is used, the resent packet will have a different sequence number and the attacker can forward it. If OSCORE is used, resent packets will have the same sequence number and the attacker must block them all until the client sends a new message with a new sequence number (not shown in Figure 4). After a while when the client has locked the door again, the attacker can deliver the delayed "unlock" message to the door, a very serious attack.

Client   Foe   Server
   |      |      |
   +----->@      |      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x9c
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
   +------------>|      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x9c
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
   <-------------+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x9c
   |      |      |
     ....   ....  
   |      |      |
   +------------>|      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x7a
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 1 (Lock)
   |      |      |
   <-------------+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x7a
   |      |      |
   |      @----->|      Code: 0.03 (PUT)
   |      | PUT  |     Token: 0x9c
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
   |      X<-----+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x9c
   |      |      |

Figure 4: Delaying request with reordering

While the second attack (Figure 4) can be mitigated by using a replay window of length zero, the first attack (Figure 3) cannot. A solution must enable the server to verify that the request was received within a certain time frame after it was sent or enable the server to securely determine an absolute point in time when the request is to be executed. This can be accomplished with either a challenge-response pattern, by exchanging timestamps between client and server, or by only allowing requests a short period after client authentication.

Requiring a fresh client authentication (such as a new TLS/DTLS handshake or an EDHOC key exchange [I-D.selander-ace-cose-ecdhe]) mitigates the problem, but requires larger messages and more processing than a dedicated solution. Security solutions based on exchanging timestamps require exactly synchronized time between client and server, and this may be hard to control with complications such as time zones and daylight saving. Wall clock time SHOULD NOT be used as it is not monotonic, may reveal that the endpoints will accept expired certificates, or reveal the endpoint's location. Use of non-monotonic clocks is not secure as the server will accept requests if the clock is moved backward and reject requests if the clock is moved forward. Even if the clocks are synchronized at one point in time, they may easily get out-of-sync and an attacker may even be able to affect the client or the server time in various ways such as setting up a fake NTP server, broadcasting false time signals to radio controlled clocks, or expose one of them to a strong gravity field. As soon as client falsely believes it is time synchronized with the server, delay attacks are possible. A challenge response mechanism where the server does not need to synchronize its time with the client is easier to analyze but require more roundtrips. The challenges, responses, and timestamps may be sent in a CoAP option or in the CoAP payload.

Remedy: The mechanisms specified in [I-D.ietf-core-echo-request-tag] or [I-D.liu-core-coap-delay-attacks] SHALL be used for controlling actuators unless another application specific challenge-response or timestamp mechanism is used.

2.3. The Response Delay and Mismatch Attack

The following attack can be performed if CoAP is protected by a security protocol where the response is not bound to the request in any way except by the CoAP token. This would include most general security protocols, such as DTLS, TLS, and IPsec, but not OSCORE. CoAP [RFC7252] uses a client generated token that the server echoes to match responses to request, but does not give any guidelines for the use of token with DTLS and TLS, except that the tokens currently "in use" SHOULD (not SHALL) be unique. The attacker performs the attack by delaying delivery of a response until the client sends a request with the same token, the response will be accepted by the client as a valid response to the later request. If CoAP is used over a reliable and ordered transport such as TCP with TLS, no messages can be delivered before the delayed message. If CoAP is used over an unreliable and unordered transport such as UDP with DTLS, other messages can be delivered before the delayed message as long as the delayed packet is delivered inside the replay window. Note that mismatches can also happen for other reasons than a malicious attacker, e.g. delayed delivery or a server sending notifications to an uninterested client.

The attack can be performed by an attacker on the wire, or an attacker simultaneously recording what the server transmits while at the same time jamming the client. The response delay and mismatch attack is illustrated in Figure 5.

Client   Foe   Server
   |      |      |
   +------------>|      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x77
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Unlock)
   |      |      |
   |      @<-----+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x77
   |      |      |
     ....   ....  
   |      |      |
   +----->X      |      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x77
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 0 (Lock)
   |      |      |
   <------@      |      Code: 2.04 (Changed)
   | 2.04 |      |     Token: 0x77
   |      |      |

Figure 5: Delaying and mismatching response to PUT

If we once again take a lock as an example, the security consequences may be severe as the client receives a response message likely to be interpreted as confirmation of a locked door, while the received response message is in fact confirming an earlier unlock of the door. As the client is likely to leave the (believed to be locked) door unattended, the attacker may enter the home, enterprise, or car protected by the lock.

The same attack may be performed on sensors, also this with serious consequences. As illustrated in Figure 6, an attacker may convince the client that the lock is locked, when it in fact is not. The "Unlock" request may be also be sent by another client authorized to control the lock.

Client   Foe   Server
   |      |      |
   +------------>|      Code: 0.01 (GET)
   | GET  |      |     Token: 0x77
   |      |      |  Uri-Path: lock
   |      |      |
   |      @<-----+      Code: 2.05 (Content)
   |      | 2.05 |     Token: 0x77
   |      |      |   Payload: 1 (Locked)
   |      |      |
   +------------>|      Code: 0.03 (PUT)
   | PUT  |      |     Token: 0x34
   |      |      |  Uri-Path: lock
   |      |      |   Payload: 1 (Unlock)
   |      |      |
   |      X<-----+      Code: 2.04 (Changed)
   |      | 2.04 |     Token: 0x34
   |      |      |
   +----->X      |      Code: 0.01 (GET)
   | GET  |      |     Token: 0x77
   |      |      |  Uri-Path: lock
   |      |      |
   <------@      |      Code: 2.05 (Content)
   | 2.05 |      |     Token: 0x77
   |      |      |   Payload: 1 (Locked)
   |      |      |

Figure 6: Delaying and mismatching response to GET

As illustrated in Figure 7, an attacker may even mix responses from different resources as long as the two resources share the same (D)TLS connection on some part of the path towards the client. This can happen if the resources are located behind a common gateway, or are served by the same CoAP proxy. An on-path attacker (not necessarily a (D)TLS endpoint such as a proxy) may e.g. deceive a client that the living room is on fire by responding with an earlier delayed response from the oven (temperatures in degree Celsius).

Client   Foe   Server
   |      |      |
   +------------>|      Code: 0.01 (GET)
   | GET  |      |     Token: 0x77
   |      |      |  Uri-Path: oven/temperature
   |      |      |
   |      @<-----+      Code: 2.05 (Content)
   |      | 2.05 |     Token: 0x77
   |      |      |   Payload: 225
   |      |      |
     ....   ....  
   |      |      |
   +----->X      |      Code: 0.01 (GET)
   | GET  |      |     Token: 0x77
   |      |      |  Uri-Path: livingroom/temperature
   |      |      |
   <------@      |      Code: 2.05 (Content)
   | 2.05 |      |     Token: 0x77
   |      |      |   Payload: 225
   |      |      |

Figure 7: Delaying and mismatching response from other resource

Remedy: If CoAP is protected with a security protocol not providing bindings between requests and responses (e.g. DTLS and TLS) the client MUST NOT reuse any tokens until the traffic keys have been replaced. The easiest way to accomplish this is to implement the Token as a counter, this approach SHOULD be followed.

2.4. The Relay Attack

Yet another type of attack can be performed in deployments where actuator actions are triggered automatically based on proximity and without any user interaction, e.g. a car (the client) constantly polling for the car key (the server) and unlocking both doors and engine as soon as the car key responds. An attacker (or pair of attackers) may simply relay the CoAP messages out-of-band, using for examples some other radio technology. By doing this, the actuator (i.e. the car) believes that the client is close by and performs actions based on that false assumption. The attack is illustrated in Figure 8. In this example the car is using an application specific challenge-response mechanism transferred as CoAP payloads.

Client   Foe         Foe   Server
   |      |           |      |
   +----->| ......... +----->|      Code: 0.02 (POST)
   | POST |           | POST |     Token: 0x3a
   |      |           |      |  Uri-Path: lock
   |      |           |      |   Payload: JwePR2iCe8b0ux (Challenge)
   |      |           |      |
   |<-----+ ......... |<-----+      Code: 2.04 (Changed)
   | 2.04 |           | 2.04 |     Token: 0x3a
   |      |           |      |   Payload: RM8i13G8D5vfXK (Response)
   |      |           |      |

Figure 8: Relay attack (the client is the actuator)

The consequences may be severe, and in the case of a car, lead to the attacker unlocking and driving away with the car, an attack that unfortunately is happening in practice.

Remedy: Getting a response over a short-range radio MUST NOT be taken as proof of proximity and therefore MUST NOT be used to take actions based on such proximity. Any automatically triggered mechanisms relying on proximity MUST use other stronger mechanisms to guarantee proximity. Mechanisms that MAY be used are: measuring the round-trip time and calculate the maximum possible distance based on the speed of light, or using radio with an extremely short range like NFC (centimeters instead of meters) that cannot be relayed through e.g. clothes. Another option is to including geographical coordinates (from e.g. GPS) in the messages and calculate proximity based on these, but in this case the location measurements MUST be very precise and the system MUST make sure that an attacker cannot influence the location estimation, something that is very hard in practice.

2.5. The Request Fragment Rearrangement Attack

These attack scenarios show that the Request Delay and Block Attacks can be used against blockwise transfers to cause unauthorized operations to be performed on the server, and responses to unauthorized operations to be mistaken for responses to authorized operations. The combination of these attacks is described as a separate attack because it makes the Request Delay Attack relevant to systems that are otherwise not time-dependent, which means that they could disregard the Request Delay Attack.

This attack works even if the individual request/response pairs are encrypted, authenticated and protected against the Response Delay and Mismatch Attack, provided the attacker is on the network path and can correctly guess which operations the respective packages belong to.

2.5.1. Completing an Operation with an Earlier Final Block

In this scenario (illustrated in Figure 9), blocks from two operations on a POST-accepting resource are combined to make the server execute an action that was not intended by the authorized client. This works only if the client attempts a second operation after the first operation failed (due to what the attacker made appear like a network outage) within the replay window. The client does not receive a confirmation on the second operation either, but, by the time the client acts on it, the server has already executed the unauthorized action.

Client   Foe   Server
   |      |      |
   +------------->    POST "incarcerate" (Block1: 0, more to come)
   |      |      |
   <-------------+    2.31 Continue (Block1: 0 received, send more)
   |      |      |
   +----->@      |    POST "valjean" (Block1: 1, last block)
   |      |      |
   +----->X      |    All retransmissions dropped
   |      |      |

(Client: Odd, but let's go on and promote Javert)

   |      |      |
   +------------->    POST "promote" (Block1: 0, more to come)
   |      |      |
   |      X<-----+    2.31 Continue (Block1: 0 received, send more)
   |      |      |
   |      @------>    POST "valjean" (Block1: 1, last block)
   |      |      |
   |      X<-----+    2.04 Valjean Promoted
   |      |      |

Figure 9: Completing an operation with an earlier final block

Remedy: If a client starts new blockwise operations on a security context that has lost packages, it needs to label the fragments in such a way that the server will not mix them up.

A mechanism to that effect is described as Request-Tag [I-D.ietf-core-echo-request-tag]. Had it been in place in the example and used for body integrity protection, the client would have set the Request-Tag option in the "promote" request. Depending on the server's capabilities and setup, either of four outcomes could have occurred:

1. The server could have processed the reinjected POST "valjean" as belonging to the original "incarcerate" block; that's the expected case when the server can handle simultaneous block transfers.
2. The server could respond 5.03 Service Unavailable, including a Max-Age option indicating how long it prefers not to take any requests that force it to overwrite the state kept for the "incarcerate" request.
3. The server could decide to drop the state kept for the "incarcerate" request's state, and process the "promote" request. The reinjected POST "valjean" will then fail with 4.08 Request Entity incomplete, indicating that the server does not have the start of the operation any more.

2.5.2. Injecting a Withheld First Block

If the first block of a request is withheld by the attacker for later use, it can be used to have the server process a different request body than intended by the client. Unlike in the previous scenario, it will return a response based on that body to the client.

Again, a first operation (that would go like “Homeless stole apples. What shall we do with him?” – “Set him free.”) is aborted by the proxy, and a part of that operation is later used in a different operation to prime the server for responding leniently to another operation that would originally have been “Hitman killed someone. What shall we do with him?” – “Hang him.”. The attack is illustrated in Figure 10.

Client   Foe   Server
   |      |      |
   +----->@      |    POST "Homeless stole apples. Wh"
   |      |      |        (Block1: 0, more to come)

(Client: We'll try that one later again; for now, we have something
more urgent:)

   |      |      |
   +------------->    POST "Hitman killed someone. Wh"
   |      |      |        (Block1: 0, more to come)
   |      |      |
   |      @<-----+    2.31 Continue (Block1: 0 received, send more)
   |      |      |
   |      @------>    POST "Homeless stole apples. Wh"
   |      |      |        (Block1: 0, more to come)
   |      |      |
   |      X<-----+    2.31 Continue (Block1: 0 received, send more)
   |      |      |
   <------@      |    2.31 Continue (Block1: 0 received, send more)
   |      |      |
   +------------->    POST "at shall we do with him?"
   |      |      |        (Block1: 1, last block)
   |      |      |
   <-------------+    2.05 "Set him free."
   |      |      |        (Block1: 1 received and this is the result)

Figure 10: Injecting a withheld first block

3. Security Considerations

The whole document can be seen as security considerations for CoAP.

4. IANA Considerations

This document has no actions for IANA.

5. References

5.1. Normative References

5.2. Informative References

Acknowledgements

The authors would like to thank Carsten Bormann, Klaus Hartke, Ari Keränen, Matthias Kovatsch, Sandeep Kumar, and András Méhes for their valuable comments and feedback.

Authors' Addresses